As part of secondary sewage treatment, primary treated sewage is treated with air or pure oxygen. In what is termed the “activated sludge” process, microorganisms utilize the oxygen to metabolize the incoming waste sewage, forming a mixture of microorganisms and sewage known as “mixed liquor.” This mixture is moved to settling tanks for concentration, thereby forming concentrated activated sludge. A majority of this sludge is returned to the activated sludge process tankage. A separate portion of this sludge, termed “waste activated sludge” (WAS), is removed from the activated sludge process and sent to a sludge handling system for further treatment and disposal. In a stable system, the daily WAS is equal to the daily conversion of sewage into microorganisms so no net increase in mixed liquor bio-mass occurs.
Referring to FIG. 1, in one typical scheme 10, the WAS is sent to a centrifuge (or other thickening apparatus) 14 for thickening, the liquids are tapped off and returned to the wastewater plant for treatment, whereas the resultant thickened sludge is sent to an anaerobic digester 16 with other sludges, where it remains for 15 days or more before being sent to a second centrifuge (or other dewatering apparatus) 18 for dewatering. Unfortunately, struvite tends to form in digester 16, and other equipment downstream because of the ammonia, magnesium and phosphorus that are present can precipitate as struvite. This struvite is impractical to harvest and also has the deleterious effect of being deposited on surfaces in the reactor 16 and plugging pipes and equipment leading from the reactor.
A further centrifuge (or other dewatering apparatus) 18 produces further dewatered sludge 20, which is either beneficially reused or disposed of, and liquids 22, which are rich in ammonia and phosphorus. It has been learned that prilled struvite can be harvested from liquids 22, by a struvite reactor 24. This prilled struvite is a marketable product that can be used as a timed release fertilizer, thereby defraying some of the costs of sewage treatment. Unfortunately, the struvite harvest requires the addition of magnesium into the process, which forms a large part of the costs of the process and reduces the profitability.